Fixing device

ABSTRACT

A fixing device in an image forming device that includes a rotating heating roller, a coil provided on the outside of the heating roller, a heating mechanism using the coil to heat the heating roller through electromagnetic induction, and a pressure roller that contacts the heating roller with pressure and, together with the heating roller, pinches and conveys a recording paper in order to fix developer that has been transferred onto the recording paper, wherein a magnetic layer is formed over the pressure roller to increase the amount of magnetic flux passing through the surface of the heating roller.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fixing device for fixing developerthat has been transferred onto a recording medium. The present inventionalso relates to an image forming device employing the fixing device.

2. Description of Related Art

An image forming device such as a laser printer is conventionallyprovided with a fixing device for fixing developer that has beentransferred onto a recording medium. A typical fixing device includes acylindrical heating roller and a pressure roller disposed parallel toand in contact with the heating roller. Developer such as toner that hasbeen transferred onto a recording medium such as paper is fixed onto therecording medium, as the medium passes between the heating roller andthe pressure roller, by the heat of the heating roller, which is heatedto about 150° C. (degrees Celsius).

Conventional fixing devices employ a halogen lamp or an electromagneticinduction type heating device as a source for heating the heatingroller. In the former type of fixing device, a halogen lamp is disposedinside a hollow heating roller. When an electric current flows throughthe halogen lamp, infrared rays emitted from the halogen lamp strike theinside walls of the heating roller, generating heat that is transferredto the surface of the roller. This construction requires fasteners orfixing mechanisms for fixing the halogen lamp in the heating roller, acomponent for connecting the halogen lamp to an electric circuit, andthe like. Since parts that do not contact the paper, including thefasteners and the connecting component, are all heated uniformly, alarge amount of heat is dissipated in the air, wasting much energy.Moreover, a longer warm-up time is required for the heating roller toreach the required temperature for fixing the developer (around 150° C.)after a current is applied to the halogen lamp.

In contrast, fixing devices that employ an electromagnetic inductiontype heating device to heat the heating roller can decrease the amountof wasted energy and can reduce the warm-up time.

For example, Japanese patent application publication No. HEI-11-297462discloses a fixing device employing an electromagnetic-induction heatingdevice. The fixing device includes a heating roller with an ironcylinder core and a pressure roller and disposes theelectromagnetic-induction heating device on the side of the heatingroller opposite the area contacting the recording medium. Theelectromagnetic-induction type heating device is configured such that acoil is supported along the outer surface of the heating roller by asupport. When an alternating current is supplied to the coil, amagnetic-field is generated around the coil and magnetic flux passingthrough the surface of the heating roller generates an eddy current.Joule heat is generated on the surface of the heating roller by the eddycurrent and the resistivity on the surface of the heating roller,thereby heating the heating roller.

SUMMARY OF THE INVENTION

However, the fixing device disclosed in Japanese patent applicationpublication No. HEI-11-297462 is problematic in that some magnetic fluxmay leak outside of the heating roller without passing through thesurface thereof. As a result, the amount of flux passing through thesurface of the heating roller is reduced by the amount of leaked flux,thereby reducing heating efficiency.

In view of the above-described drawbacks, it is an objective of thepresent invention to provide a fixing device that employs a coildisposed outside a heating member to heat the heating member throughelectromagnetic induction, wherein the fixing device achieves excellentheating efficiency by reducing the amount of magnetic flux leakingoutside the heating member without passing through the surface thereofand increasing the amount of magnetic flux passing through the surfaceof the heating member.

In order to attain the above and other objects, the present inventionprovides a fixing device for fixing a developer onto a recording medium.The fixing device includes a heating member, a magnetic-flux generatingunit, and a pressing member. The heating member has a surface and isrotatable about an axis. The axis extends in an axial direction. Themagnetic-flux generating unit includes a coil disposed outside theheating member, and a current supplying unit supplying the coil with acurrent, thereby generating magnetic flux for heating the heating memberthrough electromagnetic induction effect. The pressing member is inpressure contact with the heating member and pinches and conveys therecording medium in cooperation with the heating member in order to fixthe developer on the recording medium. The pressing member includes afirst magnetic member that increases an amount of the magnetic flux thatpasses through the surface of the heating member.

The present invention also provides an image forming device including atransferring device and a fixing device. The transferring devicetransfers a developer onto a recording medium and forms a non-fixedimage thereon. The fixing device fixes the non-fixed image on therecording medium with heat. The fixing device includes a heating member,a magnetic-flux generating unit, and a pressing member. The heatingmember has a surface and is rotatable about an axis. The magnetic-fluxgenerating unit includes a coil disposed outside the heating member, anda current supplying unit supplying the coil with a current, therebygenerating magnetic flux for heating the heating member throughelectromagnetic induction effect. The pressing member is in pressurecontact with the heating member and pinches and conveys the recordingmedium in cooperation with the heating member in order to fix thedeveloper on the recording medium. The pressing member includes a firstmagnetic member that increases an amount of the magnetic flux thatpasses through the surface of the heating member.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a perspective view showing the construction of a fixing deviceaccording to a first embodiment of the present invention;

FIG. 2( a) is a side view as viewed from an arrow IIA of FIG. 1;

FIG. 2( b) is a cross-sectional view as viewed from an arrow IIB—IIB ofFIG. 1;

FIG. 3 is a cross-sectional view as viewed from an arrow III—III ofFIGS. 1 and 2( a);

FIG. 4 is a cross-sectional view showing the construction of a fixingdevice according to a second embodiment of the present invention;

FIG. 5 is a cross-sectional view showing the construction of the fixingdevice according to the second embodiment;

FIG. 6 is a perspective view showing the construction of a fixing deviceaccording to a third embodiment of the present invention;

FIG. 7 is a cross-sectional view as viewed from an arrow VII—VII of FIG.6;

FIG. 8( a) is a cross-sectional view showing the construction of afixing device according to a fourth embodiment of the present invention;

FIG. 8( b) is an explanatory diagram (perspective view) conceptuallyshowing a second magnetic member according to the fourth embodiment;

FIG. 9 is a cross-sectional view showing the construction of a fixingdevice according to a fifth embodiment of the present invention;

FIG. 10 is a cross-sectional view showing the construction of a fixingdevice according to a sixth embodiment of the present invention;

FIG. 11 is a perspective view showing the construction of a heatingmember and a pressure roller of a fixing device according to a seventhembodiment of the present invention;

FIG. 12 is a cross-sectional view showing the construction of the fixingdevice according to the seventh embodiment;

FIG. 13 is a side view conceptually showing the construction of a colorlaser printer employing the fixing device according to the embodimentsof the present invention;

FIG. 14 is an explanatory diagram showing a modification in which a coilis wound around a heating roller such that a winding axis of the coil isparallel with an axial direction of the heating roller;

FIG. 15( a) is an explanatory diagram showing a modification in which asecond magnetic member is configured of four walls and has arectangular-tube shape;

FIG. 15( b) is an explanatory diagram showing another modification inwhich a magnetic permeability ratio of a second magnetic member isvaried along the axial direction of the heating roller; and

FIG. 15( c) is an explanatory diagram showing another modification inwhich a second magnetic member is formed dividedly to provide aplurality of magnetic member portions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A fixing device and an image forming device according to preferredembodiments of the present invention will be described while referringto the accompanying drawings wherein like parts and components aredesignated by the same reference numerals to avoid duplicatingdescription.

A fixing device according to a first embodiment of the present inventionwill be described while referring to FIGS. 1 through 3.

As shown in FIGS. 1 through 3, the fixing device 1 includes a heatingroller 2 that rotates in a rotational direction Q; a heating device 3that heats the heating roller 2 through electromagnetic induction; apressure roller 4 for pinching and conveying a recording paper P incooperation with the heating roller 2 in order to fix a toner imagecarried on the surface of the recording paper P; a driving mechanismdescribed later for driving the heating roller 2 and the pressure roller4, and attachment members 5A and 5B for fixing the fixing device 1 in apredetermined position of an image forming device or the like. Theheating roller 2 has a peripheral surface 2S and both end surfaces 7.The pressure roller 4 has a peripheral surface 4S. Hence, the fixingdevice 1 melts and fixes developer such as toner on the recording paperP by pinching and conveying the recording paper P by a nip part betweenthe heating roller 2 and the pressure roller 4. The both end surfaces 7are positioned at both ends of the heating roller 2 in an axialdirection X orthogonal to the rotational direction Q.

As shown in FIGS. 1 and 2( a), the heating device 3 includes a supportmember 6, a coil 9, and an excitation circuit 18. The support member 6is formed in a rectangular-tube shape around the both end surfaces 7 andboth imaginary side planes 8 (FIG. 2( a)). The both imaginary sideplanes 8 are defined in parallel with each other and in parallel withthe axial direction X of the heating roller 2. The both imaginary sideplanes 8 are tangent to the peripheral surface 2S. The coil 9 is woundaround the outer surface of the support member 6 to form arectangular-tube shape that surrounds a center axis of the heatingroller 2. The excitation circuit 18 supplies a current to the coil 9.

In a heating device 3 having the above-described construction, when analternating current supplied by the excitation circuit 18 flows in thecoil 9, a magnetic field is generated around the coil and magnetic fluxpasses through the peripheral surface 2S of the heating roller 2,generating an eddy current. The peripheral surface 2S of the heatingroller 2 is heated when joule heat is generated on the surface by boththe eddy current and the natural resistance of a magnetic metal layer 2a (FIG. 2( b)) forming the peripheral surface 2S.

As shown in FIGS. 2( b) and 3, the heating roller 2 is disposed in theempty space surrounded by the coil 9. In order to be heated by theheating device 3, the outer periphery of the heating roller 2 is coveredby the magnetic metal layer 2 a, which is formed of carbon steel,nickel, stainless steel, or the like. When a current flows through thecoil 9 provided around the heating roller 2, an eddy current beginsflowing along the peripheral surface 2S of the heating roller 2 to heatthe same. The heating roller 2 includes a core member 2 d formed of aninsulating resin that is nonmagnetic, nonconductive, and heat-resistant,such that the peripheral surface 2S of the heating roller 2 is heatedeffectively. The core member 2 d is formed with a hollow area 2 b. Anelastic layer 2 c is provided between the core member 2 d and themagnetic metal layer 2 a so that the recording paper P can easilyseparate from the heating roller 2.

As shown in FIG. 3, a rotational shaft 20 is fitted into the hollow area2 b one on both axial ends of the heating roller 2 and extends outwardin the axial direction. Shaft receiving portions 13 for supporting therotational shafts 20 are formed in the support member 6. The heatingroller 2 is capable of rotating since the rotational shafts 20 aresupported in the shaft receiving portions 13.

With the heating roller 2 of the above-described construction, therecording paper P is pinched between the peripheral surfaces 2S of theheating roller 2 and the peripheral surface 4S of the pressure roller 4and is conveyed by the rotations of the rollers.

As shown in FIGS. 2( b) and 3, the outer periphery of the pressureroller 4 is covered by an elastic layer 4 c, enabling the recordingpaper P to separate easily from the pressure roller 4. The pressureroller 4 includes a core member 4 d formed of an insulating resin thatis nonmagnetic, nonconductive, and heat-resistant. The core member 4 dis formed with a hollow area 4 b. A magnetic layer 4 a is providedbetween the core member 4 d and the elastic layer 4 c for increasing theamount of magnetic flux passing through the peripheral surface 2S of theheating roller 2. The magnetic layer 4 a is formed of an insulatingmaterial such as ferrite having a high magnetic permeability ratio so asto prevent a drop in the amount of magnetic flux passing through theheating roller 2 caused by the magnetic layer 4 a itself receivingmagnetic flux and dissipating heat.

As shown in FIG. 3, the pressure roller 4 is disposed to contact theperipheral surface 2S of the heating roller 2 along the axial directionX. Rotational shafts 23 are fitted into the hollow area 4 b in bothaxial ends of the pressure roller 4 and protrude outwardly in the axialdirection. Shaft receiving portions 24 are formed in the attachmentmembers 5A and 5B for rotatably supporting the rotational shafts 23. Thepressure roller 4 can rotate because the rotational shafts 23 aresupported in the shaft receiving portions 24. The pressure roller 4 canpinch the recording paper P against the peripheral surface 2S of theheating roller 2 and convey the recording paper P in the rotationaldirection.

The support member 6 is formed of an insulating resin that isnonmagnetic, nonconductive, and heat-resistant in order to increase theheating efficiency of the heating device 3.

As shown in FIGS. 1 through 2( b), the support member 6 includes sidewalls 11 and 12 and end walls 41 and 42 forming a rectangular-tube shapearound the heating roller 2 including the both end surfaces 7 and theboth imaginary side planes 8. The side walls 11 and 12 are disposedparallel to one another along the both imaginary side planes 8 in theaxial direction X. The coil 9 is wound in a rectangular-tube shapearound the side walls 11 and 12 and the end walls 41 and 42. Cornerportions 17 are formed at corners where the side walls 11 and 12 and endwalls 41 and 42 intersect with one another. The corner portions 17 arepositioned near both end surfaces 7 of the heating roller 2. A space 6S(FIG. 2( b)) is formed as a space surrounded by the side walls 11 and 12and the end walls 41 and 42.

As shown in FIG. 3, the shaft receiving portions 13 are formed in thesupport member 6 for supporting the rotational shafts 20 of the heatingroller 2 by inserting the rotational shafts 20 therein in order torotatably support the heating roller 2 and to maintain the relativepositions of the heating roller 2 and the coil 9 with good accuracy.Further, the shaft receiving portions 24 are formed in the attachmentmembers 5A and 5B for rotatably supporting the rotational shafts 23 ofthe pressure roller 4 by inserting the rotational shafts 23 therein.

As shown in FIG. 1, the corner portions 17 of the support member 6 arecurved so that the coil 9 wound around the support member 6 also forms acurve from the end walls 41 and 42 to the side walls 11 and 12.

In order to suppress increases in resistance, the coil 9 includes aplurality of twisted wires, each formed of a conductive wire covered byan insulating film such as enamel.

As described above, the coil 9 is wound in a substantiallyrectangular-tube formation around the outer surface of the supportmember 6.

The attachment members 5A and 5B are formed of an insulating resin thatis nonmagnetic, nonconductive, and heat-resistant in order to improvethe heating efficiency of the heating device 3.

As shown in FIG. 3, the attachment members 5A and 5B are disposed insidethe support member 6 near the both end surfaces 7. Each attachmentmember 5A and 5B includes a coupling portion 22 for linking with thesupport member 6, the shaft receiving portions 13 supporting therotational shaft 20 of the heating roller 2, and the shaft receivingportions 24 supporting the rotational shafts 23 of the pressure roller4.

An end portion (not shown) extending from each attachment member 5A and5B is fixed to a casing (not shown) of the image forming device or thelike. In other words, the fixing device 1 is mounted at a predeterminedposition in the device via the attachment members 5A and 5B.

The fixing device 1 includes a driving source such as a rotary motor(not shown) disposed outside the coil 9. The driving source is connectedto a rotational shaft 15 for driving the pressure roller 4. A mechanismis provided for communicating the rotational force of the driving sourceto the pressure roller 4 and to the heating roller 2.

As shown in FIG. 1, the rotational force communicating mechanism of thefixing device 1 is configured by extending one of the rotational shafts23 on the pressure roller 4 so that the rotational shaft 23 is supportedin the shaft receiving portion 24 (FIG. 3) of the attachment members SAand 5B. A protruding portion 23P protrudes out from an outer surface 5Sof the attachment member 5A. A gear 25 a is provided on the protrudingportion 23P. A gear 25 b is provided on the rotational shaft 15 so thatthe protruding portion 23P is connected to the rotational shaft 15 ofthe drive motor via the gears 25 a and 25 b. With this construction, therotational force communicated from the drive motor causes the pressureroller 4 to rotate in a direction R, while the contact between theperipheral surfaces 4S of the pressure roller 4 and the peripheralsurface 2S of the heating roller 2 causes the heating roller 2 to followthe rotation of the pressure roller 4 in the direction Q.

Next, the operations and effects of the fixing device 1 according to thefirst embodiment described above will be described.

As described above, the fixing device 1 of the first embodiment includesthe heating roller 2, the heating device 3 that employs the coil 9disposed outside of the heating roller 2 to heat the heating roller 2through electromagnetic induction; and the pressure roller 4 thatcontacts the heating roller 2 with pressure for pinching and conveyingthe recording paper P in cooperation with the heating roller 2 in orderto fix a developer image on the recording paper P. Since the pressureroller 4 is provided with the magnetic layer 4 a for increasing theamount of magnetic flux passing through the peripheral surface 2S of theheating roller 2, the fixing device 1 can improve the efficiency ofheating the heating roller 2 through electromagnetic induction. In otherwords, it is possible to increase the amount of magnetic flux passingthrough the peripheral surface 2S of the heating roller 2 by reducingthe amount of flux leaking outside the heating roller 2 without passingthrough the peripheral surface 2S. Therefore, the fixing device 1 iscapable of heating the heating roller 2 with excellent efficiency.

Further, in the fixing device 1 of the first embodiment, the magneticlayer 4 a is formed along the peripheral surface 4S of the pressureroller 4 for increasing the amount of magnetic flux that passes throughthe peripheral surface 2S of the heating roller 2. Accordingly, thefixing device 1 has excellent efficiency in heating the heating roller 2and achieves excellent production efficiency, without adding extracomponents for disposing a separate magnetic member on the pressureroller 4.

Further, since the magnetic layer 4 a according to the first embodimentis formed of an insulating material, efficiency for heating the heatingroller 2 can be improved without losing heating efficiency caused by themagnetic layer 4 a itself receiving magnetic flux and dissipating heat.

Further, the heating roller 2 is positioned within the space 6Ssurrounded by the coil 9 (the side walls 11 and 12 and the end walls 41and 42) in the fixing device 1. By providing the heating roller 2 withinthe magnetic field in the space 6S, variations in the amount of magneticflux passing through the surface of the heating roller 2 can be reduced,thereby improving the efficiency for heating the heating roller 2.

Next, a fixing device 31 according to a second embodiment of the presentinvention will be described with reference to FIGS. 4 and 5.

FIGS. 4 and 5 show the construction of the fixing device 31 according tothe second embodiment. FIG. 4 is a cross-sectional view corresponding toFIG. 3 of the first embodiment while FIG. 5 is a cross-sectional viewcorresponding to FIG. 2( b) according to the first embodiment.

Since the fixing device 31 of the second embodiment is similar inconstruction to the fixing device 1 of the first embodiment, only thefeatures of the second embodiment will be described, and like parts andcomponents are designated by the same reference numerals to avoidduplicating description.

As shown in FIGS. 4 and 5, the fixing device 31 is provided with a firstmagnetic member 33 that is supported on a supporting member 32 in thehollow area 44 b of the pressure roller 44. Further, both ends of thesupporting member 32 protrude outside of the hollow area 44 b and arefixed to the attachment members 45A and 45B.

The first magnetic member 33 includes a plurality of magnetic members 33a, 33 b, 33 c, 33 d, and 33 e that are disposed at predeterminedintervals along an axis 10 of the heating roller 2.

In order to improve the efficiency of heating the peripheral surface 2Sof the heating roller 2 and to reduce variation in temperature on theperipheral surface 2S, the first magnetic member 33 is configured sothat the end portions along the axis 10 of the heating roller 2 have ahigher magnetic permeability ratio than that of the center portion. Inother words, the magnetic permeability ratio of the magnetic members 33b and 33 d that are outside the center magnetic member 33 c is higherthan that of the magnetic member 33 c and the magnetic permeabilityratio of the outermost magnetic members 33 a and 33 e is higher thanthat of the magnetic members 33 b and 33 d.

The first magnetic member 33 is formed of ferrite or another insulatingmaterial having a high magnetic permeability ratio so that the firstmagnetic member 33 itself does not receive magnetic flux and dissipateheat, which would lead to a loss in the amount of magnetic flux passingthrough the peripheral surface 2S of the heating roller 2.

As in the first embodiment, the elastic layer 44 c is formed over theperipheral surface 44S of the pressure roller 44. Inside the elasticlayer 44 c are the magnetic layer 44 a, and the core member 44 d havingthe hollow area 44 b.

A belt 35 is looped around the peripheral surface 44S of the pressureroller 44 on one end with respect to an axis 37 of the pressure roller44 and a peripheral surface (not shown) of the rotational shaft of thedriving source for communicating the rotational driving force of thedriving source to the pressure roller 44.

The peripheral surface 44S of the pressure roller 44 is in pressurecontact with the peripheral surface 2S of the heating roller 2 along theaxis thereof. The peripheral surface 44S of the pressure roller 44 onboth axial ends is supported in shaft receiving portions 36 that areformed in the attachment members 45A and 45B. The peripheral surfaces44S of the pressure roller 44 and heating roller 2 pinch the recordingpaper P and convey the recording paper P in the direction of rotation.

The driving mechanism of the fixing device 31 includes a driving source(not shown) having a rotational shaft. The driving source is disposedoutside the coil 9 wound around the support member 6. A mechanism isprovided for communicating a rotational force of the driving source tothe pressure roller 44 and the heating roller 2 as described below.

The rotation communicating mechanism of the fixing device 31 includesthe driving source, such as a rotary motor, provided with the rotationalshaft (not shown), the belt 35 looped around the rotational shaft of thedriving source and the peripheral surface 44S of the pressure roller 44,the attachment members 45A and 45B that rotatably support the rotationalshafts 20 of the heating roller 2 and the peripheral surface 44S of thepressure roller 4, and the pressure roller 44 that contacts the heatingroller 2 with pressure. Since the belt 35 moves when the rotationalshaft of the driving source rotates, the rotational force of the drivingsource is communicated to the pressure roller 44 via the belt 35. Therotational force of the pressure roller 44 is further communicated tothe heating roller 2, and the heating roller 2 and the pressure roller44 rotate together.

Next, the operations and effects of the fixing device 31 according tothe second embodiment described above will be described.

In the fixing device 31 of the second embodiment, the first magneticmember 33 is supported on the supporting member 32 inside the hollowarea 44 b formed in the pressure roller 44. Hence, it is possible toincrease the amount of magnetic flux passing through the peripheralsurface 2S of the heating roller 2 by decreasing the amount of magneticflux leaking out of the heating roller 2 without passing through theperipheral surface 2S, thereby improving the efficiency for heating theheating roller 2.

The hollow area 44 b is provided in the pressure roller 44 foraccommodating the first magnetic member 33. This improves productionefficiency because a desired shape of the first magnetic member 33 canbe selected as long as the first magnetic member 33 can be accommodatedwithin the size and shape of the core member 44 d, and because the firstmagnetic member 33 can be replaced.

As described above, the first magnetic member 33 in the fixing device 31of the second embodiment is configured of the plurality of magneticmembers 33 a, 33 b, 33 c, 33 d, and 33 e disposed along the axis 10 ofthe heating roller 2. Further, the magnetic permeability ratio of themagnetic members 33 b and 33 d that are farther outside the centermagnetic member 33 c with respect to the axis 10 is greater than that ofthe magnetic member 33 c, while the magnetic permeability ratio of theoutermost magnetic members 33 a and 33 e is set higher than that of themagnetic members 33 b and 33 d. In this way, the peripheral surface 2Sof the heating roller 2 can be heated uniformly across the axis 10.

Further, since the first magnetic member 33 includes the plurality ofmagnetic members 33 a, 33 b, 33 c, 33 d, and 33 e, the surfacetemperature of the heating roller 2 and the efficiency for heating theperipheral surface 2S can easily be controlled by varying the magneticpermeability ratios of each magnetic member.

Since the first magnetic member 33 in the fixing device 31 according tothe second embodiment is formed of an insulating material, a loss inheating efficiency caused by the first magnetic member 33 itselfreceiving magnetic flux and dissipating heat can be avoided, therebyimproving the efficiency of heating the heating roller 2.

Next, an fixing device according to a third embodiment of the presentinvention will be described with reference to FIGS. 6 and 7.

FIG. 6 is a perspective view showing the construction of a fixing device51 according to the third embodiment. FIG. 7 is a cross-sectional viewas viewed from an arrow VII—VII of FIG. 6. Since the fixing device 51 ofthe third embodiment is similar in construction to the fixing device 1of the first embodiment, only the features of the third embodiment willbe described, and components common to the fixing device 1 and thefixing device 51 will not be described in detail.

As shown in FIGS. 6 and 7, the fixing device 51 includes the heatingroller 2 that rotates in a circumferential direction Q, the heatingdevice 3 for heating the heating roller 2 through electromagneticinduction, two rollers 52 and 53 that rotate about different rotationalaxes that are substantially parallel with each other, a pressure belt 54that is looped around the two rollers 52 and 53 and contacts theperipheral surface 2S of the heating roller 2 for pinching the recordingpaper P against the heating roller 2 and conveying the recording paper Pwhile moving circularly around the rollers 52 and 53, a drivingmechanism for driving the pressure belt 54 and the heating roller 2, andthe attachment members 65A and 65B for fixing the fixing device 51 at apredetermined position in the image forming device. In the fixing device51, the recording paper P on which an image in toner or anotherdeveloper has been transferred is pinched and conveyed by the nip partbetween the heating roller 2 and the pressure belt 54. The heat of theheating roller 2 melts the developer, fixing the developer on therecording paper P.

The pressure belt 54 is formed of a synthetic resin that is bothflexible and insulating, so that the pressure belt 54 can pinch andconvey the recording paper P together with the heating roller 2; therecording paper P can easily separate from the pressure belt 54; and thepressure belt 54 itself does not receive magnetic flux and dissipateheat, which can cause a drop in magnetic energy used to heat the heatingroller 2.

As shown in FIG. 7, the rollers 52 and 53 are arranged in opposition tothe heating roller 2 with the pressure belt 54 interposed therebetween.The rollers 52 and 53 are configured of core members 52 b and 53 bformed of an insulating material, and magnetic layers 52 a and 53 aformed over the surfaces of the core members 52 b and 53 b. The magneticlayers 52 a and 53 a are formed of ferrite or another insulatingmaterial to avoid receiving magnetic flux and dissipating heat, therebypreventing heating efficiency from deteriorating.

As shown in FIG. 6, rotational shafts 55A and 55B protrude from bothaxial ends of the rollers 52. Similarly, rotational shafts 56A and 56B(56B is not shown) protrude from both axial ends of the rollers 53.Shaft receiving portions (not shown) are formed in the attachmentmembers 65A and 65B for rotatably supporting the rotational shafts 55Aand 55B and rotational shafts 56A and 56B. With this construction, therollers 52 and 53 can pinch the recording paper P against the peripheralsurface 2S of the heating roller 2 and convey the recording paper P inthe direction of rotation.

The fixing device 51 also includes a driving source (not shown) such asa rotary motor for driving the heating roller 2, the rollers 52 and 53,and the pressure belt 54. The driving source is connected to therotational shaft 15 that is positioned outside the coil 9. A rotationcommunicating mechanism communicates the rotational force from thedriving source to the pressure belt 54 and the heating roller 2.

The rotational shafts 55A and 55B of the roller 52 are supported inshaft receiving portions formed in the attachment members 65A and 65Band protrudes from the outer surface of the attachment members 65A and65B, respectively. The rotational shafts 56A and 56B (56B is not shown)of the roller 53 are also supported in different shaft receivingportions formed in the attachment members 65A and 65B, but do notprotrude from the attachment members 65A and 65B. The rotationcommunicating mechanism of the fixing device 51 is configured byconnecting the protruding portion of the rotational shaft 55A to therotational shaft 15 of the driving motor via the gears 25 a and 25 b.With this construction, the rotational force from the driving motor inthe direction R is transferred to the pressure belt 54, which movescircularly around the outer surfaces of the rollers 52 and 53. Since theperipheral surface 2S of the heating roller 2 is in pressure contactwith the pressure belt 54, the heating roller 2 rotates in the directionQ along with the pressure belt 54.

Next, the operations and effects of the fixing device 51 according tothe third embodiment described above will be described.

In the fixing device 51 of the third embodiment, the magnetic layers 52a and 53 a are formed over the peripheral surfaces of the rollers 52 and53, and the pressure belt 54 is looped around the rollers 52 and 53,thereby improving the efficiency for heating the heating roller 2through electromagnetic induction. In other words, the fixing device 51can increase the amount of magnetic flux passing through the peripheralsurface 2S of the heating roller 2 by reducing the amount of flux thatleaks outside of the heating roller 2 without passing through theperipheral surface 2S, thereby obtaining a fixing device 51 havingexcellent efficiency in heating the heating roller 2.

Since the magnetic layers 52 a and 53 a are formed over the peripheralsurface of the rollers 52 and 53 in the fixing device 51 of the thirdembodiment, there is no need to provide extra parts for increasing theamount of magnetic flux passing through the heating roller 2, therebyachieving good production efficiency.

Next, a fixing device according to a fourth embodiment of the presentinvention will be described with reference to FIGS. 8( a) and 8(b). FIG.8( a) is a cross-sectional view showing a fixing device 61 according tothe fourth embodiment and corresponds to FIG. 2( b) of the firstembodiment. FIG. 8( b) is an explanatory diagram (perspective view)showing a shape of a second magnetic member 62. Since the fixing device61 of the fourth embodiment is similar in construction to the fixingdevice 1 of the first embodiment, only the features of the fourthembodiment will be described, and like parts and components aredesignated by the same reference numerals to avoid duplicatingdescription.

As shown in FIG. 8( a), the fixing device 61 according to the fourthembodiment is configured similarly to the fixing device 1 of the firstembodiment with the support member 6 disposed around the heating roller2 and the coil 9 wound about the support member 6, so that the heatingroller 2 is positioned in the space 6S surrounded by the support member6 and the coil 9.

The fixing device 61 further includes the second magnetic member 62surrounding the coil 9 for increasing the amount of flux that passesthrough the peripheral surface 2S of the heating roller 2 by decreasingthe amount of flux leaking outside the coil 9. As shown in FIG. 8( b),the second magnetic member 62 is configured of side walls 62C and 62Ddisposed to surround the coil 9 and the heating roller 2 (the coil 9 isnot shown in FIG. 8( b)). In other words, the coil 9 and the heatingroller 2 are interposed between the side walls 62C and 62D. The sidewalls 62C and 62D are fixed to the support member 6 by a mounting piece(not shown). The second magnetic member 62 is formed of ferrite oranother magnetic material that is insulating and has high magneticpermeability so that the second magnetic member 62 itself does notdissipate heat, which can reduce the amount of magnetic flux passingthrough the peripheral surface 2S of the heating roller 2.

As in the first embodiment, the elastic layer 4 c is formed along theperipheral surface 4S of the pressure roller 4, while the inner portionof the pressure roller 4 includes the core member 4 d formed with thehollow area 4 b, the magnetic layer 4 a, and the like.

Next, the operations and effects of the fixing device 61 according tothe fourth embodiment described above will be described.

Since the fixing device 61 according to the fourth embodiment includesthe magnetic layer 4 a formed over the peripheral surface 4S of thepressure roller 4 and the second magnetic member 62 surrounding the coil9, the fixing device 61 can further increase the amount of magnetic fluxpassing through the peripheral surface 2S of the heating roller 2 byfurther decreasing the amount of flux leaking outside of the heatingroller 2, thereby improving the efficiency for heating the heatingroller 2.

Since the second magnetic member 62 is formed of an insulating materialin the fixing device 61 of the fourth embodiment, heating efficiency isnot decreased by the fixing device 61 itself receiving magnetic flux anddissipating heat, thereby improving the efficiency for heating theheating roller 2.

Next, a fixing device according to a fifth embodiment of the presentinvention will be described with reference to FIG. 9. FIG. 9 is across-sectional view showing the construction of a fixing device 71according to the fifth embodiment. FIG. 9 corresponds to FIG. 2( b) ofthe first embodiment. Since the fixing device 71 of the fifth embodimentis similar in construction to the fixing device 1 of the firstembodiment, only the features of the fifth embodiment will be described,and components common to the fixing device 1 and the fixing device 71will not be described in detail.

As shown in FIG. 9, the fixing device 71 according to the fifthembodiment is configured similarly to the fixing device 1 of the firstembodiment with a support member 76 disposed around the heating roller 2and a coil 79 wound about the support member 76, so that the heatingroller 2 is positioned in a space 7S surrounded by the support member 76and the coil 79.

In the fifth embodiment, the support member 76 includes side walls 72and 73 that oppose each other with the heating roller 2 interposedtherebetween. The portion of the side walls 72 and 73 indicated by aportion S1 from the points intersected by a horizontal plane HP passingthrough a central axis CA of the heating roller 2 to the ends nearer thepressure roller 4 is formed in a curve that follows the peripheralsurface 2S of the heating roller 2 so as to approach the magnetic layer4 a of the pressure roller 4. The remaining portion indicated by aportion S2 of the side walls 72 and 73 from the points intersected bythe horizontal plane HP away from the pressure roller 4 are formedparallel to each other.

The coil 79 is wound around the outer surfaces of the side walls 72 and73. Hence, the portion S1 of the coil 79 from a point intersected by thehorizontal plane HP toward the pressure roller 4 slants toward themagnetic layer 4 a of the pressure roller 4.

Next, the operations and effects of the fixing device 71 according tothe fifth embodiment described above will be described. Since the coil79 of the fixing device 71 according to the fifth embodiment slantstoward the magnetic layer 4 a of the pressure roller 4 below thehorizontal plane HP, the amount of magnetic flux passing through theperipheral surface 2S of the heating roller 2 is larger than when thecoil 79 is separated farther from the magnetic layer 4 a, therebyfurther improving the efficiency for heating the heating roller 2.

Next, a fixing device according to a sixth embodiment of the presentinvention will be described with reference to FIG. 10. FIG. 10 is across-sectional view showing the construction of a fixing device 81according to the sixth embodiment and corresponds to FIG. 7 of the thirdembodiment. Since the fixing device 81 of the sixth embodiment issimilar in construction to the fixing device 51 (FIG. 7) of the thirdembodiment, only the features of the sixth embodiment will be described,and components common to the fixing device 51 and the fixing device 81will not be described in detail.

As shown in FIG. 10, a fixing device 81 according to the sixthembodiment includes two rollers 82 and 83 and a pressure belt 84 loopedaround the rollers 82 and 83. The recording paper P on which developerhas been transferred is pinched between the heating roller 2 and thepressure belt 84 and conveyed as the pressure belt 84 moves circularlyaround the rollers 82 and 83. At this time, the developer is fixed ontothe recording paper P by the heating roller 2.

The first magnetic member 33 is supported on the supporting member 32and is disposed inside the pressure belt 84 and faces the heating roller2 in order to increase the amount of flux that passes through theperipheral surface 2S of the heating roller 2. The supporting member 32protrudes outside of the pressure belt 84 in the axial directionorthogonal to the direction R in which the pressure belt 84 moves. Bothends of the supporting member 32 are fixed to the attachment members 85Aand 85B (85A is not shown), similarly as shown in FIG. 4 of the secondembodiment.

The pressure belt 84 is configured of an elastic layer 84 b and amagnetic layer 84 a. The elastic layer 84 b is formed on the outsidesurface of the pressure belt 84. The elastic layer 84 b is formed ofrubber or another elastic material so that the recording paper P pinchedand conveyed between the pressure belt 84 and the heating roller 2 caneasily separate from the pressure belt 84. The magnetic layer 84 a isformed on the inside surface of the pressure belt 84 for improvingheating efficiency by increasing the amount of magnetic flux passingthrough the heating roller 2. The magnetic layer 84 a is formed offerrite or another insulating material that does not receive flux anddissipate heat so as not to reduce heating efficiency.

Next, the operations and effects of the fixing device 81 according tothe sixth embodiment described above will be described.

By disposing the first magnetic member 33 inside the pressure belt 84and opposing the heating roller 2, the fixing device 81 of the sixthembodiment can increase the amount of flux passing through theperipheral surface 2S of the heating roller 2 by decreasing the amountleaking outside of the heating roller 2, thereby improving theefficiency for heating the heating roller 2. Further, by forming themagnetic layer 84 a along the inner surface of the pressure belt 84, itis possible to further increase the amount of magnetic flux passingthrough the peripheral surface 2S of the heating roller 2, therebyfurther improving efficiency for heating the heating roller 2.

Next, a fixing device according to a seventh embodiment of the presentinvention will be described with reference to FIGS. 11 and 12. FIG. 11is a perspective view showing the construction of a heating roller and apressure roller in a fixing device 91 according to the seventhembodiment. FIG. 12 is a cross-sectional view of the fixing device 91according to the seventh embodiment and corresponds to FIG. 2( b) of thefirst embodiment. Since the fixing device 91 of the seventh embodimentis similar in construction to the fixing device 1 of the firstembodiment, only the features of the seventh embodiment will bedescribed, and components common to the fixing device 1 and the fixingdevice 91 will not be described in detail.

As shown in FIGS. 11 and 12, the fixing device 91 includes a guidemember 93, a heating member 92, the pressure roller 4, the heatingdevice 3 (FIG. 12), and the attachment members 5A and 5B (the attachmentmember 5A is not shown in FIG. 12).

The guide member 93 is substantially shaped like a half cylinder andincludes support portions 93A and 93B and an interposed portion 94. Theinterposed portion 94 is supported by the support portions 93A and 93Bbetween the same. The support portions 93A and 93B are formed of aninsulating resin that is nonmagnetic, nonconductive, and heat resistant,in order to increase the heating efficiency of the heating device 3. Theinterposed portion 94 has a smooth surface 94S in contact with theheating member 92, in order to facilitate sliding of the heating member92.

The heating member 92 is formed of a cylindrical film that is slidablydisposed over the peripheral surface of the guide member 93. Thepressure roller 4 is disposed parallel to and in contact with theperipheral surface 92S of the heating member 92 for pinching andconveying the recording paper P in cooperation with the heating member92. The heating device 3 is for heating the heating member 92 throughelectromagnetic induction. The attachment members 5A and 5B are forfixing the fixing device 91 at a predetermined position in the imageforming device.

In the fixing device 91 having the above-described construction, therecording paper P on which developer such as toner has been transferredis pinched and conveyed by the nip part between the heating member 92and the pressure roller 4, whereby the developer is melted and fixedonto the recording paper P.

The heating member 92 is formed of a conductive and magnetic thin metalfilm, such as a carbon steel, nickel, or stainless steel film having athickness of 50 μm, to be heated by the heating device 3.

The heating member 92 is fitted over the semi-cylindrical guide member93 and is capable of sliding over the peripheral surface of the guidemember 93. The heating member 92 is disposed so that the peripheralsurface 92S of the heating member 92 contacts the pressure roller 4along an axial direction X. The rotation of the pressure roller 4 istransferred to the heating member 92 causing the heating member 92 torotate around the peripheral surface of the guide member 93 in therotational direction Q in FIG. 11.

As in the first embodiment, one of the rotational shafts 23 on thepressure roller 4 is connected to the rotational shaft of a drive motor(not shown) via gears. With this construction, rotational force from thedrive motor in the rotational direction R is transferred to the pressureroller 4. Since the peripheral surface 4S of the pressure roller 4contacts the peripheral surface 92S of the heating member 92, arotational force in the rotational direction Q in FIG. 11 is transferredto the heating member 92.

As in the first embodiment, the heating device in the present embodimentincludes the coil 9 wound in a rectangular-tube shape around the heatingmember 92 with a gap formed therebetween. The coil 9 is formed aroundthe periphery of the heating member 92 that includes both end portions95 of the heating member 92 and both imaginary side planes 98 that areparallel to the axial direction X.

As in the first embodiment, the pressure roller 4 includes the coremember 4 d formed of an insulating resin that is nonmagnetic,nonconductive, and heat-resistant. The core member 4 d has the hollowarea 4 b. The magnetic layer 4 a is formed over the core member 4 d forincreasing the amount of magnetic flux passing through the peripheralsurface 92S of the heating roller 92, and the elastic layer 4 c isformed over the magnetic layer 4 a. The magnetic layer 4 a is formed ofan insulating material such as ferrite having a high magneticpermeability ratio so as to prevent a drop in the amount of magneticflux passing through the heating roller 92 caused by the magnetic layer4 a itself receiving magnetic flux and dissipating heat.

As in the first embodiment, the heating device 3 of the fixing device 91includes a driving source (not shown) having a rotational shaft outsideof the coil 9, which is wound around the support member 6. A rotationcommunicating mechanism is configured to transfer the rotational forcefrom the driving source to the pressure roller 4 and to the heatingmember 92.

Next, the operations and effects of the fixing device 91 according tothe seventh embodiment described above will be described.

By forming the heating member 92 as a film, the fixing device 91according to the seventh embodiment can reduce the heating capacityrequired to raise the temperature of the heating member 92 to the fixingtemperature, enabling the fixing temperature to be reached quickly afteractuating the heating device 3 and reducing temperature variation in theheating member 92 in order to perform efficient heating. Further, byproviding the magnetic layer 4 a along the peripheral surface 4S of thepressure roller 4, it is possible to increase the amount of magneticflux passing through the heating member 92 by reducing the amount offlux leaking outside of the heating member 92, thereby improving theefficiency of heating the heating member 92.

Next, an image forming device employing the fixing device will bedescribed with reference to FIG. 13. FIG. 13 is an explanatory diagramshowing the construction of a color laser printer 101 employing thefixing device 1, 31, 51, 61, 71, 81, or 91 according to the firstthrough seventh embodiments described above.

As shown in FIG. 13, the color laser printer 101 includes a main casing102 and, within the main casing 102, a feeder unit 104 for feedingsheets of a recording paper 103, an image forming unit 105 for formingpredetermined images on the recording paper 103 supplied from the feederunit 104, and the like.

Sheets of the recording paper 103 are stacked on a paper supply tray 106disposed in the feeder unit 104. The topmost sheet of the recordingpaper 103 stacked on the paper supply tray 106 is supplied one sheet ata time by the rotation of a feed roller 107 and conveyed to the imageforming unit 105 by conveying rollers 108 and registration rollers 109.

The image forming unit 105 includes a scanning unit 110 for forminglatent images based on predetermined image data by scanning a laserlight over the surface of a photosensitive belt 122 described later, aprocessing unit 111 for transferring developer such as toner onto thephotosensitive belt 122, an intermediate transfer belt mechanism 112, atransfer roller 113, and a fixing unit 114.

The scanning unit 110 includes a laser light emitting element, a polygonmirror, and a plurality of lenses and reflecting mirrors (not shown). Inthe scanning unit 110, the laser light emitting element emits a laserbeam based on predetermined image data. The laser beam passes through orreflects off of the reflecting mirrors and lenses and is irradiated in ahigh-speed scanning motion onto the surface of the photosensitive belt122 in a photosensitive belt mechanism 116 described later.

The processing unit 111 includes developing cartridges 115, thephotosensitive belt mechanism 116, and a Scorotron charging device 117.

In the present embodiment, four developing cartridges 115 are providedto include a yellow developing cartridge 115Y for supplying yellowtoner, a magenta developing cartridge 115M for supplying magenta toner,a cyan developing cartridge 115C for supplying cyan toner, and a blackdeveloping cartridge 115K for supplying black toner.

Each of the developing cartridges 115 includes a toner accommodatingsection for accommodating a positively charged toner of the colorsyellow, magenta, cyan, and black, respectively. Each developingcartridge 115 also includes a supply roller (not shown) that rotates tosupply toner onto a developing roller 118. The toner carried on thedeveloping roller 118 is regulated at a uniform thin layer by athickness-regulating blade (not shown). At this time, the toner ispositively charged and is, thus, attracted to the surface of thedeveloping roller 118 by electrostatic force.

The photosensitive belt mechanism 116 includes a first photosensitivebelt roller 119, a second photosensitive belt roller 120, and a thirdphotosensitive belt roller 121 that are disposed in a triangulararrangement. The photosensitive belt 122 is looped around these threerollers.

The photosensitive belt 122 is formed of a synthetic resin such aspolyethylene terephthalate (PET) on the surface of which aluminum hasbeen deposited. An organic photosensitive layer is provided on thesurface of the photosensitive belt 122.

When a driving source (not shown) drives the second photosensitive beltroller 120 to rotate, the photosensitive belt 122 moves circularlycounterclockwise in FIG. 12. While rotating circularly, thephotosensitive belt 122 is exposed to laser light from the scanning unit110, whereby an electrostatic latent image is formed on the surface ofthe photosensitive belt 122. Next, the latent image on thephotosensitive belt 122 comes into contact with the developing rollers118 carrying positively charged toner, and the latent image is developedinto a toner image.

When the second photosensitive belt roller 120 is driven to rotate, thephotosensitive belt 122 moves circularly and the first photosensitivebelt roller 119 and third photosensitive belt roller 121 follow therotation of the second photosensitive belt roller 120.

The intermediate transfer belt mechanism 112 is disposed adjacent to thephotosensitive belt mechanism 116. The intermediate transfer beltmechanism 112 includes a first intermediate transfer belt roller 123disposed in confrontation with the second photosensitive belt roller 120through the photosensitive belt 122 and an intermediate transfer belt126, a second intermediate transfer belt roller 124 disposed inconfrontation with the transfer roller 113 described later through theintermediate transfer belt 126, a third intermediate transfer beltroller 125 disposed in a position forming a triangle with the firstintermediate transfer belt roller 123 and the second intermediatetransfer belt roller 124, and the intermediate transfer belt 126 loopedaround the three intermediate transfer belt rollers.

The intermediate transfer belt 126 is formed of a heat-resistantsynthetic resin in which have been dispersed conductive particles suchas carbon. The intermediate transfer belt 126 moves circularly clockwisein FIG. 13, while contacting the photosensitive belt 122 at a transferposition A. The toner image formed on the photosensitive belt 122 istransferred to the intermediate transfer belt 126 at the transferposition A. In the present embodiment, four colors of toner are used.Therefore, as the photosensitive belt 122 continues moving circularly, acleaning roller 135 connected to a charge remover cleans thephotosensitive belt 122. Subsequently, the Scorotron charging device 117recharges the photosensitive belt 122. Next, an electrostatic latentimage for the next color is formed on the photosensitive belt 122 and isdeveloped into a toner image. This toner image is superimposed over thetoner image previously transferred onto the intermediate transfer belt126. A four-color image is transferred onto the intermediate transferbelt 126 by repeating this process for each of the four colors.

The transfer roller 113 is movably disposed at a position opposing thesecond intermediate transfer belt roller 124 with the intermediatetransfer belt 126 interposed therebetween, so that the transfer roller113 can contact or separate from the surface of the intermediatetransfer belt 126. When the recording paper 103 is being conveyed, thetransfer roller 113 is moved into contact with the intermediate transferbelt 126, and a predetermined transfer bias is applied to the transferroller 113. The four-color image formed on the intermediate transferbelt 126 is transferred all at once onto the recording paper 103 as therecording paper 103 passes between the intermediate transfer belt 126and the transfer roller 113.

At this time, the color image transferred on the recording paper 103 isnot fixed yet. Next, the recording paper 103 is conveyed to the fixingunit 114 for fixing the color image on the recording paper 103.

The fixing unit 114 has a construction equivalent to any of the fixingdevices 1, 31, 51, 61, 71, 81, or 91 according to the first throughseventh embodiments described above. The fixing unit 114 includes aheating roller 127 and a pressure roller 128 for fixing the color imageon the recording paper 103 as the recording paper 103 passestherebetween.

After the color image is fixed on the recording paper 103 in the fixingunit 114, conveying rollers 129 convey the recording paper 103 towarddischarge rollers 130. The discharge rollers 130 discharge the recordingpaper 103 onto a discharge tray 131.

Since the image forming device 101 according to the eighth embodimentdescribed above employs the fixing unit 114 having a constructionequivalent to that described in any of the first through seventhembodiments and, hence, having good heating efficiency, the imageforming device 101 can uniformly fix the developer on the recordingpaper 103 to obtain an image that has been reproduced and fixed well.

While the invention has been described in detail with reference to thespecific embodiment thereof, it would be apparent to those skilled inthe art that various changes and modifications may be made thereinwithout departing from the spirit of the invention.

For example, in the fixing device 1 according to the first embodiment(FIG. 1), the coil 9 is wound around the heating roller 2, such that thewinding axis of the coil 9 is perpendicular to the axial direction X ofthe heating roller 2. However, as shown in a conceptual diagram of FIG.14, a coil 99 may also be wound around the heating roller 2, such thatthe winding axis of the coil 99 is parallel with the axial direction Xof the heating roller 2.

Further, in the fixing device 31 of the second embodiment (FIG. 4), theplurality of magnetic members 33 a, 33 b, 33 c, 33 d, and 33 e making upthe first magnetic member 33 is provided on the top surface of thesupporting member 32 and the magnetic permeability ratio grows largerfrom the center magnetic member 33 c toward the magnetic members 33 aand 33 e on the ends. However, the first magnetic member 33 may beprovided as a single member, rather than being divided into a pluralityof magnetic members, while having a magnetic permeability ratio thatgrows larger from the center toward the ends.

In the fixing device 51 according to the third embodiment (FIG. 7), tworollers 52 and 53 having magnetic layers 52 a and 53 a are disposedinside the pressure belt 54. However, the number of rollers is notlimited to two, but may be increased to a larger number. In that case, amagnetic layer may be provided in selected rollers for improving heatingefficiency, rather than providing the magnetic layer in all of therollers.

In the fixing device 61 according to the fourth embodiment (FIGS. 8( a)and 8(b)), the second magnetic member 62 is disposed to surround thecoil 9. However, the second magnetic member 62 may be extended tosurround the pressure roller 4 as well. This construction can furtherimprove efficiency for heating the heating roller 2.

In the fixing device 61 according to the fourth embodiment (FIGS. 8( a)and 8(b)), the second magnetic member 62 is configured of the two sidewalls 62C and 62D. As shown in FIG. 15( a), however, a second magneticmember 162 is configured of end walls 162A and 162B and side walls 162Cand 162D all made from a magnetic material, thereby forming arectangular-tube shape surrounding the coil 9 and the heating roller 2.The second magnetic member 162 has the same effect as the secondmagnetic member 62, that is, the second magnetic member 162 can increasethe amount of flux that passes through the peripheral surface 2S of theheating roller 2. Alternatively, the end walls 162A and 162B may be madefrom a non-magnetic material, in which case the end walls 162A and 162Bare provided for connecting the side walls 162C and 162D.

In the fixing device 61 according to the fourth embodiment (FIGS. 8( a)and 8(b)), the magnetic permeability ratio of the second magnetic member62 is constant along the axial direction of the heating roller 2. Asshown in FIG. 15( b), however, the magnetic permeability ratio of asecond magnetic member 262 is varied along the axial direction of theheating roller 2. In other words, the magnetic permeability ratio of thesecond magnetic member 262 is higher on the both end portions than inthe center portion with regard to the axial direction of the heatingroller 2. With this construction, it is possible to improve efficiencyof heating the heating roller 2 and to reduce temperature variations onthe peripheral surface 2S of the heating roller 2.

In another modification shown in FIG. 15( c), a second magnetic member362 is formed dividedly to provide a plurality of magnetic memberportions 362 a through 362 j that is arranged along the axial directionof the heating roller 2. The second magnetic member 362 is configured sothat the end portions along the axial direction of the heating roller 2have a higher magnetic permeability ratio than that of the centerportion. In other words, the magnetic permeability ratio of the magneticmember portions 362 b and 362 d that are outside the center magneticmember portion 362 c is higher than that of the magnetic member portion362 c and the magnetic permeability ratio of the outermost magneticmember portions 362 a and 362 e is higher than that of the magneticmember portions 362 b and 362 d. The same goes for the magnetic memberportions 362 f through 362 j. With this construction, it is possible toobtain effect similar to that obtained by the second magnetic member 262shown in FIG. 15( b).

Further, although the coil 9 in the embodiments described above is woundaround the outer periphery of the support member 6, the coil 9 mayinstead be wound along the inner surface of the support member 6.

Further, the thickness and surface area for portions of the first andsecond magnetic members opposing the heating roller 2 may be variedalong the axial direction X of the heating roller 2 in order to increasethe amount of flux passing through the heating roller 2 and improveheating efficiency.

While the image forming device according to the eighth embodiment formsfour-color images, the image forming device may also be a type thatforms single-color images. Further, the image forming device accordingto the eighth embodiment described above forms a color image on onesurface of the recording paper 103. However, after fixing the colorimage on one side of the recording paper 103, the recording paper 103may be inverted to form a color image on the reverse side thereof.

1. A fixing device for fixing a developer onto a recording medium, thedevice comprising: a heating member having a surface and rotatable aboutan axis, the axis extending in an axial direction; a magnetic-fluxgenerating unit including: a coil disposed outside the heating member;and a current supplying unit supplying the coil with a current, therebygenerating magnetic flux for heating the heating member throughelectromagnetic induction effect; and a pressing member in pressurecontact with the heating member and pinching and conveying the recordingmedium in cooperation with the heating member in order to fix thedeveloper on the recording medium, the pressing member including a firstmagnetic member that increases an amount of the magnetic flux thatpasses through the surface of the heating member, wherein the pressingmember is formed with a hollow portion; wherein the first magneticmember is disposed inside the hollow portion, the first magnetic memberbeing provided as a separate member from the pressing member, the firstmagnetic member not rotating with the pressing member; wherein the coilis disposed outside the pressing member, wherein a hollow space isdefined inside the coil, and wherein the heating member is positioned inthe hollow space of the coil; and wherein the coil is disposed at aposition that is shifted, by a predetermined length, toward the firstmagnetic member from a position at which the axis of the heating memberis positioned.
 2. The fixing device as claimed in claim 1, wherein thepressing member comprises a pressure roller rotatable about a rotationalaxis.
 3. The fixing device as claimed in claim 1, wherein the pressingmember includes: at least two rollers rotatable about differentrotational axes that are substantially parallel with each other; and apressure belt looped around the at least two rollers and movablecircularly around the at least two rollers; and wherein the hollowportion is formed between the at least two rollers and inside a loop ofthe pressure belt.
 4. The fixing device as claimed in claim 3, whereinthe first magnetic member is disposed at a position confronting theheating member through the pressure belt.
 5. The fixing device asclaimed in claim 1, wherein the first magnetic member extends along theaxial direction of the heating member; wherein the first magnetic memberhas a center portion and end portions with respect to the axialdirection; and wherein the first magnetic member has a larger magneticpermeability ratio on the end portions than in the center portion. 6.The fixing device as claimed in claim 5, wherein the first magneticmember includes a plurality of magnetic members arranged along the axialdirection of the heating member.
 7. The fixing device as claimed inclaim 1, wherein the first magnetic member is formed of an insulatingmaterial.
 8. The fixing device as claimed in claim 1, further comprisinga second magnetic member disposed to surround the coil.
 9. The fixingdevice as claimed in claim 8, wherein the second magnetic member extendsalong the axial direction of the heating member and has a center portionand end portions with respect to the axial direction; and wherein thesecond magnetic member has a greater magnetic permeability ratio on theend portions than in the center portion.
 10. The fixing device asclaimed in claim 9, wherein the second magnetic member includes a pairof elongated walls disposed to interpose the coil therebetween, eachelongated wall extending along the axial direction; and wherein eachelongated wall has a greater magnetic permeability ratio on the endportions than in the center portion.
 11. The fixing device as claimed inclaim 9, wherein the second magnetic member is formed dividedly toprovide a plurality of magnetic member portions arranged along the axialdirection of the heating member.
 12. The fixing device as claimed inclaim 8, wherein the second magnetic member is formed of an insulatingmaterial.
 13. The fixing device as claimed in claim 1, wherein theheating member and the pressing member are aligned with each other in analignment direction; wherein the heating member is a substantiallycylindrical member having a peripheral surface surrounding an axis andtwo end surfaces substantially perpendicular to the axis; and whereinthe coil encloses the heating member in a cross section of the heatingmember and the coil, the cross section being perpendicular to thealignment direction.
 14. The fixing device as claimed in claim 13,further comprising attachment members at positions where each attachmentmember confronts a corresponding one of the end surfaces, and whereinthe coil is wound around the attachment members.
 15. An image formingdevice comprising: a transferring device transferring a developer onto arecording medium and forming a non-fixed image thereon; and a fixingdevice for fixing the non-fixed image on the recording medium with heat,the device including: a heating member having a surface and rotatableabout an axis; a magnetic-flux generating unit including: a coildisposed outside the heating member; and a current supplying unitsupplying the coil with a current, thereby generating magnetic flux forheating the heating member through electromagnetic induction effect; anda pressing member in pressure contact with the heating member andpinching and conveying the recording medium in cooperation with theheating member in order to fix the developer on the recording medium,the pressing member including a first magnetic member that increases anamount of the magnetic flux that passes through the surface of theheating member, wherein the pressing member is formed with a hollowportion; wherein the first magnetic member is disposed inside the hollowportion, the first magnetic member being provided as a separate memberfrom the pressing member, the first magnetic member not rotating withthe pressing member; wherein the coil is disposed outside the pressingmember, wherein a hollow space is defined inside the coil, and whereinthe heating member is positioned in the hollow space of the coil; andwherein the coil is disposed at a position that is shifted, by apredetermined length, toward the first magnetic member from a positionat which the axis of the heating member is positioned.
 16. A fixingdevice for fixing a developer onto a recording medium, the devicecomprising: a heating member having a surface and rotatable about anaxis, the axis extending in an axial direction; a magnetic-fluxgenerating unit including: a coil disposed outside the heating member;and a current supplying unit supplying the coil with a current, therebygenerating magnetic flux for heating the heating member throughelectromagnetic induction effect; and a pressing member in pressurecontact with the heating member and pinching and conveying the recordingmedium in cooperation with the heating member in order to fix thedeveloper on the recording medium, the pressing member including a firstmagnetic member that increases an amount of the magnetic flux thatpasses through the surface of the heating member, wherein the firstmagnetic member is formed of an insulating material, wherein a hollowspace is defined inside the coil, and wherein the heating member ispositioned in the hollow space of the coil; and wherein the coil isdisposed at a position that is shifted, by a predetermined length,toward the first magnetic member from a position at which the axis ofthe heating member is positioned.
 17. The fixing device as claimed inclaim 16, wherein the pressing member comprises a main body in pressurecontact with the heating member; and wherein the main body isconstituted by the first magnetic member formed of a magnetic material.18. The fixing device as claimed in claim 16, wherein the pressingmember has a peripheral surface in pressure contact with the heatingmember; and wherein the first magnetic member comprises a magnetic layerformed along the peripheral surface of the pressing member.
 19. Thefixing device as claimed in claim 16, wherein the pressing memberincludes: at least two rollers rotatable about different rotational axesthat are substantially parallel with each other; and a pressure beltlooped around the at least two rollers and movable circularly around theat least two rollers.
 20. The fixing device as claimed in claim 19,wherein each roller has a peripheral surface in contact with thepressure belt; and wherein the first magnetic member includes a magneticlayer formed along the peripheral surface of each roller.
 21. The fixingdevice as claimed in claim 19, wherein the first magnetic memberincludes a magnetic layer formed along a substantially entire peripheryof the pressure belt.
 22. A fixing device for fixing a developer onto arecording medium, the device comprising: a heating member having asurface and rotatable about an axis, the axis extending in an axialdirection; a magnetic-flux generating unit including: a coil disposedoutside the heating member; and a current supplying unit supplying thecoil with a current, thereby generating magnetic flux for heating theheating member through electromagnetic induction effect; and a pressingmember in pressure contact with the heating member and pinching andconveying the recording medium in cooperation with the heating member inorder to fix the developer on the recording medium, the pressing memberincluding a first magnetic member that increases an amount of themagnetic flux that passes through the surface of the heating member,wherein the pressing member is formed with a hollow portion; wherein thefirst magnetic member is disposed inside the hollow portion, the firstmagnetic member being provided as a separate member from the pressingmember, the first magnetic member not rotating with the pressing member;wherein the coil is disposed outside the pressing member; wherein ahollow space is defined inside the coil, and wherein the heating memberis positioned in the hollow space of the coil; wherein the heatingmember and the pressing member are aligned with each other in analignment direction; wherein the heating member is a substantiallycylindrical member having a peripheral surface surrounding an axis andtwo end surfaces substantially perpendicular to the axis; wherein thecoil encloses the heating member in a cross section of the heatingmember and the coil, the cross section being perpendicular to thealignment direction; the fixing device further comprising attachmentmembers at positions where each attachment member confronts acorresponding one of the end surfaces; and wherein the coil is woundaround the attachment members.
 23. An image forming device comprising: atransferring device transferring a developer onto a recording medium andforming a non-fixed image thereon; and a fixing device for fixing thenon-fixed image on the recording medium with heat, the device including:a heating member having a surface and rotatable about an axis; amagnetic-flux generating unit including: a coil disposed outside theheating member; and a current supplying unit supplying the coil with acurrent, thereby generating magnetic flux for heating the heating memberthrough electromagnetic induction effect; and a pressing member inpressure contact with the heating member and pinching and conveying therecording medium in cooperation with the heating member in order to fixthe developer on the recording medium, the pressing member including afirst magnetic member that increases an amount of the magnetic flux thatpasses through the surface of the heating member, wherein the pressingmember is formed with a hollow portion; wherein the first magneticmember is disposed inside the hollow portion, the first magnetic memberbeing provided as a separate member from the pressing member, the firstmagnetic member not rotating with the pressing member; wherein the coilis disposed outside the pressing member; wherein a hollow space isdefined inside the coil, and wherein the heating member is positioned inthe hollow space of the coil; wherein the heating member and thepressing member are aligned with each other in an alignment direction;wherein the heating member is a substantially cylindrical member havinga peripheral surface surrounding an axis and two end surfacessubstantially perpendicular to the axis; wherein the coil encloses theheating member in a cross section of the heating member and the coil,the cross section being perpendicular to the alignment direction; thefixing device further comprising attachment members at positions whereeach attachment member confronts a corresponding one of the endsurfaces; and wherein the coil is wound around the attachment members.24. A fixing device for fixing a developer onto a recording medium, thedevice comprising: a heating member having a surface and rotatable aboutan axis, the axis extending in an axial direction; a magnetic-fluxgenerating unit including: a coil disposed outside the heating member;and a current supplying unit supplying the coil with a current, therebygenerating magnetic flux for heating the heating member throughelectromagnetic induction effect; and a pressing member in pressurecontact with the heating member and pinching and conveying the recordingmedium in cooperation with the heating member in order to fix thedeveloper on the recording medium, the pressing member including a firstmagnetic member that increases an amount of the magnetic flux thatpasses through the surface of the heating member, wherein the firstmagnetic member is formed of an insulating material; wherein a hollowspace is defined inside the coil, and wherein the heating member ispositioned in the hollow space of the coil; wherein the heating memberand the pressing member are aligned with each other in an alignmentdirection; wherein the heating member is a substantially cylindricalmember having a peripheral surface surrounding an axis and two endsurfaces substantially perpendicular to the axis; wherein the coilencloses the heating member in a cross section of the heating member andthe coil, the cross section being perpendicular to the alignmentdirection; the fixing device further comprising attachment members atpositions where each attachment member confronts a corresponding one ofthe end surfaces; and wherein the coil is wound around the attachmentmembers.